The present invention relates to a method for thermal treatment of metal workpieces, in particular for nitriding or nitrocarburizing iron alloy articles such as high alloy steels. The invention furthermore relates to the use of an apparatus for performing such a method.
Metal workpieces are subjected to a thermochemical heat treatment for producing defined workpiece properties, e.g. high resistance to wear or sufficient corrosion resistance. In nitriding and nitrocarburizing, the result of the heat treatment is that the case of the workpiece is enriched with nitrogen and/or carbon in order to provide the workpiece with the required mechanical and chemical properties at the surface and in the case.
In nitriding, e.g. in a gas atmosphere containing ammonia, the surface layer or case is enriched with nitrogen in that the ammonia (NH.sub.3) contained in a reaction gas generally breaks down into nitrogen (N) and hydrogen (H) at temperatures greater than 500.degree. C. under the catalytic effects of the surface of the workpieces that are to be subjected to nitriding. The ammonia molecule is adsorbed and gradually broken down at the workpiece surface, whereby the required nitrogen is released in its atomic form and is available for dissolving in the iron and for forming iron nitride (Fe.sub.x N). In nitrocarburizing, in addition, the case is simultaneously enriched with carbon. Atomic carbon (C) diffuses through the surface of the workpiece into the case in an analogous manner.
In general, the case is of particular importance in terms of the properties the treated workpieces must have. In addition to hexagonal .epsilon.-nitride (Fe.sub.2-3 N), it can also have cubic face-centered .gamma.'-nitride (Fe.sub.4 N) and furthermore can have nitrides from nitride-forming alloy elements, e.g. chromium nitride, molybdenum nitride, manganese nitride, titanium nitride, niobium nitride, tungsten nitride, vanadium nitride, and aluminum nitride.
In particular with chromium and/or nickel alloy iron articles, as the content of the alloy elements increases, more passivation occurs that manifests itself, e.g., in local variations in hardening, known as soft spots. Passivation makes it more difficult for the nitrogen to transition from the gas phase into the material as described in the foregoing; consequently the results of subsequent nitriding or nitrocarburizing are limited.
Known in prior art therefore are a number of methods for influencing the surface condition of workpieces in terms of effecting an improved nitriding or nitrocarburizing result. For instance, a series of articles by H. Sidan points out that satisfactory nitriding of highly alloyed iron articles (which are consequently inclined to passivation) requires the passive layer to be destroyed ("Nitriding Rust-Proof and Acid-Proof Steels", Technische Rundschau (1966) 24, p. 913 ff., Technische Rundschau (1966) 28 pp. 3-7, Technische Rundschau (1966) 42, pp. 33 -37 and 45). Additional known measures are, e.g. pre-oxidizing or oxinitriding. In the latter, an oxygen carrier is added to a reaction gas, which results in exterior oxidation of the treated workpieces. Depending on the composition of the reaction gas used, during the further course of nitriding Fe.sub.3 O.sub.4 layers form that are permeable for nitrogen and lead to the destruction of any passive layers. Usage of the activating effect of oxidizing gasses on nitrogen absorption in highly alloyed iron articles, e.g. stainless steels, is covered, e.g. by Spies, et. al., in their article entitled "Gas Oxinitriding of High Alloy Steels", HTM 52 (1997) 6, pp. 342-349. On the other hand, in an essay by Stiles et. al., the authors suggest heating high chromium content steels in the absence of oxygen in a reducing atmosphere ("Accelerating the Gas Nitriding Process by Pretreating in the Reactive Gas Phase", HTM 53, (1998) 4, pp. 211 through 219).
However, the disadvantage of all of these measures is either that the nitride layer produced by nitriding or nitrocarburizing is not sufficiently uniform or that conditions for forming the nitride layer are required that are technically unfeasible or would be very difficult to attain.
The object of the invention is therefore to create a method for heat treating metal workpieces in which a substantially uniform nitride layer can be obtained even in workpieces made of highly alloyed iron articles.